So you want to heat a cold frame in Zone 4b where the temperatures go down to some 20 degrees below zero at times. You don't have a source of power and do not appear to want to consider it. I checked the calendar and it is not April 1st. I think you may want to do some more research through a local university and get their input.

It's a cold frame. That means it's not heated. There are things you can do to help maintain better conditions. Placement is important for sun conditions... against a house will give it more protection and warmth. The surface it's on can help to absorb heat. You can put gallon jugs of hot water inside the frame to help heat it. If you're getting a really cold night, you can cover it with blankets. I can go on and on. But in the end, if you plan to heat it with a space heater then you're treating it like a greenhouse and should research that.

I would use soil heating cables under the soil. This video shows you how to do it. Some of them are designed to be used with a thermostat (so you can set the desired temperature) and others will not have a thermostat (you just plug them in). This heats up your soil and will speed up germination and help to keep the roots warm. You should not be trying to use this with warm weather crops such as tomatoes, as they will not get warm enough in your zone. But I have always had excellent luck with cabbages, broccoli, spinach, lettuce, cilantro, carrots, radishes, kohlrabi, etc. If you need more heat on a cold night a warm blanket would do, or even a burlap bag filled with leaves. Anything to throw over the top of the cold frame to keep heat in. Tammy, www.growyourownfoodanywhere.com

OP said they don't have outdoor power. Can you get battery soil cables? There are kerosene and propane greenhouse heaters available but they might be overkill for a cold frame.

Is it even possible to buy battery-powered soil cables? Are they even available? If so, where? I Looked online for soil cables, and all of the ones I found you have to plug into some sort of outlet. Also you said that kerosene/propane heaters would most likely overheat a cold frame. I could see how this would be an issue for stuff that doesn't like hot weather (i.e. any leaf green thing, root veggies, cauliflower, cabbage etc.) However what about stuff that likes hot weather (peppers, tomatoes, etc.)

You need to tell us what you want to use this frame for and its dimensions. In my experience they're usually used for protecting hardy seedlings over winter, protecting relatively hardy vegetables over winter, for extending the season at the beginning and end and for growing things like melons which need more heat than my climate provides. The latter are grown in summer when solar heat through the glass provides the heat. The same would apply to peppers and tomatoes. And you'd need to choose small cultivars to fit in a frame. They're not really a method of growing truly tender things in winter. For that you need a proper greenhouse. Cold frames also need constant monitoring. Unless you have automatic openers they need opening and closing and also covering with something when necessary.

Sorry everyone, I missed the part about no electricity. There are no battery powered soil cables that I know of. It sounds like you are maybe going to try to use this for tomatoes, peppers, heat loving plants. Have you ever thought about growing them inside your home? The temperature should be in the proper range. You will need lights, but they shouldn't be any more expensive to buy and operate than building your cold frame. Especially if you use fluorescent lights. Here is a blog post I wrote about growing under light. From what you are saying I think this would maybe be your best option. Tammy

Any heat that is built up during the day in a cold frame and even in a greenhouse will quickly disappear after the sun goes down. That is why you need supplemental heating.

Somebody asked what size the cold frame would be. I'd go with what is considered the traditional size, which according to what I found online is....

A cold frame can be any width that the lights will cover and any length or height. Traditional home garden cold frames measure 4 to 6 feet front to back and are 8 to 12 feet long. They are laid out with the long dimension running east to west. The frame should be just tall enough to clear the crops you plan to grow.

However somebody also said that any heat the cold frame accumulates during the day is going to vanish once the sun goes down. So now I have two problems, heating the cold frame and RETAINING the heat it accumulated during the day at night. I realize in a traditional cold frame set up, it's not heated; however since it gets so cold where I live, I have to heat it so my plants don't freeze.

Yes I'd be willing to grow starter plants indoors however it's again not an option, it's just not feasible. That doesn't mean I'm unwilling to do so, it's just not possible.

Even if I could grow starter plants indoors, the problem I have had is, they keep stretching for the light (yes I kept the light close to the seedlings). The other problem is that even when they weren't stretching for the light, the two problems I have had are.....

1.The soil dries out too fast.

2. They will germinate and grow, but once they reach about 2-3 weeks of age they sit there and do nothing (i.e. I don't see any new growth)

The other thing is I really don't plan to grow plants from start to finish in the cold frame; I just plan to grow starter plants in the cold frame. Most of the stuff I would be growing is cold weather stuff. The purpose of the cold frame would be not only to prevent them frost/freezing but also to prevent them from becoming dormant. Below is a list of the stuff I'd grow in the cold frame (no I'm not saying I'd grow everything l listed in the cold frame all at the same time, it's just for reference).

1.Kai Lan (a mini version of broccoli)

2.Napa Cabbage

3.Asian Greens (komatsuna, mibuna, misome, tatsoi, pak choi, bok choi)

4.Sorrel (another leafy green thing)

5Fennel

6.Radicchio

7.Leeks

8.Escarole

9.Cauliflower

10. Celeriac (Idk if I wouwould grow this)

I've looked at other methods for heating it (electric blankets, aquarium heaters) but again those all would require an outlet so those aren't options either. The problem with using jugs of hot water, is I would constantly have to be refilli them with fresh hot water, as the water would cool down; I also don't know how long the water would stay hot.

Two points I see from your additional info. One is that is very big for a cold frame. The footprint is as big as a small greenhouse. The other is that most of your vegetables are very hardy types and wouldn't need any additional heat in most climates. Can you tell us where you are located? I'm still not clear how you intend to use this frame. Are you wanting to use it to sow seed very early in the year? Or are you wanting to use it to grow hot weather crops like tomatoes and peppers in summer?

Meanwhile, the kerosene heaters I'm thinking of look like This.

Yes you can use containers painted black with water in them. They should be sealed containers. No, you do not have to refill them. The water will heat during the day and dissipate its heat during the night. I don't think you will have much room for these if you are making the height of the cold frame just above the plants, but you could probably put some. I would usually suggest a heavy blanket inside the roof, but you could probably put one on the outside of this roof just as easy. Here are some more suggestions for the unheated winter greenhouse. I agree that the list of plants you have supplied can probably be grown in most areas in an unheated greenhouse.

Have you checked with your local extension office? They are much more familiar with your climate than any of us, simply because they live there too. This is a free service. They may also have recommendations for varieties of the plants you have chosen that are more adaptable to your climate. If you don't know how to find them just do a search for county extension office "my town".

I am located in northern Michigan (upper part of the lower peninsula); my growing zone is 4b/5a (I don't know which it is exactly; when I look up my growing zone online, some sites say it's 4b, other sites say it's 5a). The last frost date for my area is end of May; however for cold weather stuff they can be planted at beginning of May (sometimes mid-to-late April if we have an early spring).

The purpose of the cold frame would be to extend my growing season with cold weather vegetables. However the main purpose of it would be growing starter plants (cold weather starter plants). Somebody said that I could insulate it by painting buckets black, putting lids on them, and filing them with hot water. However wouldn't I need supplemental heating on days that it's cloudy? I realize the cold frame would be sufficiently warm on sunny days, but what do I do on cloudy days? Unfortunately where I live, most days it's cloudy in late winter/early spring.

I would prefer to start stuff in the cold frame in March. Would this even be possible? I could germinate the seeds indoors, then transfer them to the cold frame.

Seems to me you just need to get on and do it and see how it goes. A bunch of strangers on the internet can't guess exactly what your conditions will be and solve every issue in advance. Start growing something and see if or when you need supplemental heating.

I have heard that bales of hay are effective insulators and throw off heat as they compost. I am interested in this discussion as I too have a cold frame that I am determined to learn how to use. I pressed my husband to make it for me, and it's been sitting idle for two years! I had intended to use it to start seedlings more successfully than inside and even grow lettuce. What has discouraged me has been the realization of how labor intensive it will be to raise and lower the glass (now plexiglass since the glass broke) tops to avoid freezing and frying the plants. Wintersowing in plastic milk jugs is so much easier by comparison. Just put out and forget about them.

Perhaps the best way for you and me to proceed with our cold frames is to focus on one or two objectives. Here are mine: how should I grow lettuce? And how should I start Love Lies Bleeding (amaranthus) so that it can grow large enough in my garden in the summer?

My apologies for piggybacking on your post. I will post separately if any prefer. I just thought that we are both grappling with the same problem. (I am in zone 6b, which at the moment is 0 degrees!) My cold frame is a wooden box 4' x 8' x 2' h against a south-facing stone wall, covered with loose plexiglass sheets.

I made a cold frame instead of winter sowing this year. It is constructed in the style of a low tunnel instead of a glass frame. I laid down a wood box frame, built my hoops, covered them in insect netting, then covered that in row cover fabric which is air and water permeable. The purpose of mine is to cold stratify and start perennial seeds. On warm days I can untuck the row cover at the ends for ventilation, leaving the screen material in place to keep out weed seeds, pests, and blowing leaves. For more control you could use a plastic cover but then you'll have to water by hand and pay more attention to temperature. All my seeds are in containers, I would definitely need a plastic cover if i was growing straight in the ground, to control soil moisture. Heating a structure like this does seem like it would be futile without a lot of tinkering or daily control.

If you want to get crazy, maybe you could find an old hot water heater that doesnt leak, connect it to a loop of pipes buried in the soil, making it a heat exchange system. Then you can retrofit it to handle a fire or a propane burner under it every day to heat the water tank. I dont expect anyone to do this but I would sure like to see the result lol.

I agree with Flora just start doing it and see how it goes. When I first started using cold frames and tunnels I was always worried about it. Now I just let things be without heating, thermometers, christmas lights and blankets etc. The only other protection I use is agribon directly over the plants during very cold spells. Over time you will get the techniques that you need.

It might help to read Eliot Coleman's books on season extending techniques. I have a high tunnel (plastic covered/wood frame) and it is amazing how warm it gets during the day when the sun is out. What I have learned is the soil inside doesn't freeze even though outside it might be zero degrees and it gets zero degrees inside at night. From the list that another poster provided, those are cold hardy plants that can take some cold temperature extremes. Local farmers start using supplemental heat when they want an early tomato crop but they don't try to grow tomatoes all year. You also need to understand that during the darkest winter days, plants aren't getting enough sunlight so they tend to stop growing. Eliot calls this the Persephone Period. Depending on what you are trying to grow you might need both heat and light. (New local hydroponic greenhouses are using a lot of light.) Local market gardeners provide extra cold protection for their plants by using Agribon. I managed to keep rosemary alive for two winters here in zone 5 NH by swaddling the plant with Agribon, removing it in the spring. By providing enough cold protection you can probably grow spinach in the winter but not lettuce. I have had lettuce roots survive and start growing in the spring. Kale, coriander, and claytonia might die but seed and/or roots survive to start growing in the spring. Keep in mind that even snow provides some insulation. Johnny's Seeds website might also have some helpful information about when to sow plants for fall/winter and when to sow in the spring for earliest crops.

My parents garden in the northern LP so I am somewhat familiar with the conditions the OP is in. Starting tender new plants in March, when there is still a high possibility of snow accumulation and hard freezes down to 10-20 degrees fahrenheit, is ambitious even with a cold frame. Hours of sunlight are also a consideration, although the days will lengthen quickly. I would wait until a few weeks after the equinox, maybe early April, and use two techniques that others have already described: use the blackened water containers as heat sinks, and use blankets as insulation depending on the forecasted overnight lows. Black containers tend to absorb what heat is there even on cloudy days, so I wouldn't worry too much about cloud cover. Even so, in early April you are risking the possible loss of your starter plants. If it is important to be safe and not lose anything, I would not start plants even in a cold frame until the last week of April. For what it is worth, there is enough sunlight up there to get really good harvests even if you wait until the end of May to plant your veggies.

I am in 5b, without a fire or electricity, won't be able to do much. Those black jugs of water and solar this and that just aren't enough for the long winter nights. (Dec-Feb)